The five-time Gold Medalist and 11-time world champion made the admission today in an interview with British TV personality Sir Michael Parkinson.
Speculation over Thorpe’s sexuality first surfaced in 2009, two years after Thorpe ended a supposedly romantic relationship with fellow swimmer Amanda Beard.
But Beard reportedly told her agent later on that she and Thorpe were only ever friends and the relationship was concocted to shield his true sexual preference.
"I've thought about this for a long time. I'm not straight," the 31-year-old told Parkinson. "And this is only something that very recently, we're talking the past two weeks, I've been comfortable telling the closest people around me, exactly that.
"I've wanted to [come out] for some time but I couldn't, I didn't feel as though I could. What happened was I felt the lie had become so big that I didn't want people to question my integrity."
In his 2012 autobiography, Thorpe wrote that “all my sexual experiences have been straight” and that he’s “attracted to women.”
Throughout the interview, the swimmer also detailed his battle with depression, drugs and alcohol, which caused him to check into a rehabilitation unit earlier this year.
So far, the reaction to Thorpe's admission on social media has been strongly positive.
"Ian Thorpe we're proud of YOU Regardless of sexuality, achievements, medals or anything else,” wrote one Twitter fan, while another added:
"Life's much brighter on this side of the closet @IanThorpe. Welcome to being you."